Thank you for your input, Roger !
I was not really thinking of going as high as a Rio M at 195 L ! My original dilemma was between the 2008 Rio S ( 175 L) and a last year's Small model of 150 L .

I think, on the basis of what you told me, that I can safely rule out the 2007 150 L, but wouldn't the 2008 SMALL at 175 L still be O.K. for me, even as a quasi beginner, at 78 Kg ( 172 lbs) ?
Would the 4 cm difference in width , as compared to the M , significantly change the handling, especially the stability, for the better or for the worse ?
Ideally, if there is not much of a difference, I'd prefer a S, because it would snugly fit into my car and cut down on the time required for loading and fastening it on the roof-top every time , as well avoid the risk of scratching the body of my brand new car in the process.
On the other hand, if there IS a substantial disadvantage for me in going down to a 175 L, I'd be ready to sacrifice the advantage of fitting the board inside the car.